Until recent settlement, Australia and New Zealand were isolated for millions of years, and their ecosystems have evolved to cope with unique climate and biological circumstances (Kemp, 1981; Nix, 1981). Despite large year-to-year climatic variability, many Australian terrestrial species have quite limited ranges of long-term average climate, on the order or 1-2°C temperature and 20% in rainfall (Hughes et al., 1996; Pouliquen-Young and Newman, 1999). Thus, these ecosystems are vulnerable to climatic change, as well as invasion by exotic animals and plants.

Rapid land clearance and subsequent land-use change have been occurring as a result of human activity over the past 500-1,000 years in New Zealand (McGlone, 1989; Wilmshurst, 1997) and, in Australia, subsequent to Aboriginal arrival tens of thousands of years ago—and especially since European settlers arrived 200 years ago. This has led to loss of biodiversity in many ecosystems as well as loss of some ecosystems as a whole. One of the major impacts has been an increase in weedy species in both countries. This is likely to continue and be exacerbated by climate change. Land-use change also has led to fragmentation of ecosystems and to salinization through rising water tables. These trends can inhibit natural adaptation to climate change via the dispersal/migration response. Systems therefore may be more vulnerable, and some might become extinct. For example, Mitchell and Williams (1996) have noted that habitat that is climatically suitable for the long-lived New Zealand kauri tree Agathis australis under a 4°C warming scenario would be at least 150 km from the nearest extant population. They suggest that survival of this species may require human intervention and relocation. Similar problems have been identified by Pouliquen-Young and Newman (1999) in relation to fragmented habitat for endangered species in the southwest of western Australia.

Many of the region's wetlands, riverine environments, and coastal and marine systems also are sensitive to climate variations and changes. A key issue is the effect on Australia's coral reefs of greenhouse-related stresses in addition to nonclimatic features such as overexploitation and increasing pollution and turbidity of coastal waters from sediment loading, fertilizers, pesticides, and herbicides (Larcombe et al., 1996).

In Australia, some 50% of the forest cover in existence at the time of European settlement still exists, although about half of that has been logged (Graetz et al., 1995; State of the Environment, 1996). Pressures on forests and woodlands as a whole are likely to decrease as a result of recent legislation relating to protection of forests in some Australian states, and as interest in carbon sequestration increases. In New Zealand (Taylor and Smith, 1987), 25% of the original forest cover remains, with 77% in the conservation estate, 21% in private hands, and 2% state owned. Legal constraints on native wood production mean that only about 4% currently is managed for production, and clear-felling without replacement has virtually ceased.

The present temperature range of 25% of Australian Eucalyptus trees is less than 1°C in mean annual temperature (Hughes et al., 1996). Similarly, 23% have ranges of mean annual rainfall of less than 20% variation. The actual climate tolerances of many species are wider than the climate envelope they currently occupy and may be affected by increasing CO₂ concentrations, which change photosynthetic rates and water-use efficiency (WUE) and may affect the temperature response (Curtis, 1996). Such changes from increasing CO₂ would be moderated by nutrient stress and other stressors that are prevalent across Australian forests. Nevertheless, if present-day boundaries even approximately reflect actual thermal or rainfall tolerances, substantial changes in Australian native forests may be expected with climate change. Howden and Gorman (1999) suggest that adaptive responses would include monitoring of key indicators, flexibility in reserve allocation, increased reserve areas, and reduced fragmentation.

In a forested area in western Australia that is listed as one of 25 global "biodiversity hotspots" for conservation priority by Myers et al. (2000), Pouliquen-Young and Newman (1999) used the BIOCLIM program (Busby, 1991) to generate a climatic envelope from the present distribution of species. They assessed the effects of three incremental temperature and rainfall scenarios on three species of frogs, 15 species of endangered or threatened mammals, 92 varieties of the plant genus Dryandra, and 27 varieties of Acacia in the southwest of western Australia. The scenarios were based on the spatial pattern of change from the CSIRO RCM at 125-km resolution, scaled to the IS92 global scenarios. For plant species, suitability of soils also was considered. The results indicate that most species would suffer dramatic decreases in range with climate warming; all of the frog and mammal species studied would be restricted to small areas or would disappear with 0.5°C global-average warming above present annual averages, as would 28% of the Dryandra species and one Acacia. At 2°C global average warming, 66% of the Dryandra species, as well as all of the Acacia, would disappear. Adaptation opportunities were considered minimal, with some gain from linking present conservation reserves and reintroducing endangered species into a range of climatic zones.

Studies of the current distribution of New Zealand canopy trees in relation to climate suggest that major range changes can be expected with warming (Whitehead et al., 1992; Leathwick et al. 1996; Mitchell and Williams, 1996). Trees in the highly diverse northern and lowland forests (e.g., Beilschmiedia tawa) are likely to expand their ranges southward and to higher altitudes. The extensive upland Nothofagus forests are likely to be invaded by broad-leafed species. Few tree species are confined to cool southern climates; those that are have a wide altitudinal range available for adjustment of their distribution, so no extinctions are expected. Most concern centers on the ability of tree species to achieve new distributions rapidly enough in a fragmented landscape, as well as invasion of natural intact forests by exotic tree, shrub, and liana species that are adapted to warm temperate or subtropical climates.

In Australia, rangelands are important for meat and wool production. In their natural state, rangelands are adapted to relatively large short-term variations in climatic conditions (mainly rainfall and temperature). However, they are under stress from human activity, mostly as a result of animal production, introduced animals such as rabbits, inappropriate management, and interactions between all of these factors (Abel et al., 2000). These stresses, in combination with climatic factors, have led to problems of land degradation, salinization, and woody weed invasion and subsequent decreases in food production. In some cases, native dominant species (mostly plants) have been replaced by exotic species, leading to a decrease in population of many native animal species. Woody weed invasion also has changed the fire regime through formation of "thickets" that do not allow fires through, partly as a result of the fire resistance of some species (Noble et al., 1996). Some Australian rangelands also are vulnerable to salinization resulting from rising water tables from irrigation and loss of native vegetation (see Section 12.3).

New Zealand rangelands are used predominantly for sheep grazing. Intensive use of indigenous grasslands and shrublands on land cleared of trees in the 19th century has increased vulnerability to invasion, especially by woody weeds (pine, broom, gorse, etc.) and herbaceous weeds (hawkweed, thistles, and subtropical grasses). Weed invasions are unlikely to further increase the susceptibility of the system to climatic disruptions but could themselves be accelerated by warming or increased climatic variability. Fire is now strictly regulated, although rangeland fires remain a serious problem, especially in ENSO drought years. Rangelands—in particular, in drier areas of eastern South Island—have many problems, including animal and plant pests and declining profitability of farming, leading to a decline in management and fertilizer inputs.

Increased CO₂ is likely to have beneficial effects for native pastures, with possible nitrogen limitation and increased subsoil drainage (Howden et al., 1999d). Runoff and groundwater recharge also could increase (Krysanova et al., 1999). This could lead to increased salinization problems in areas that are susceptible. However, decreases in rainfall in excess of about 10% at the time of CO₂ doubling would dominate over the CO₂ fertilization effect and lead to a decline in pasture productivity. This is more likely in the latest climate change scenarios that are based on coupled AOGCM results. Howden et al. (1999d) conclude that a doubling of CO₂ concentrations will result in only limited changes in the distribution of C₃ and C₄ grasses and that such changes will be moderated by warmer temperatures.

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